1. Technical Field
The invention relates to an apparatus and method of monitoring the sag in energized electrical conductors such as power lines so as to assure safe clearance from the ground. More particularly, the invention relates to an apparatus and method for measuring the sag of energized electrical conductors such as power lines in real time as it changes with varying electrical load on the line as well as varying environmental conditions causing thermal expansion where the sag monitor assists in the determination of the maximum power transmission feasible through such conductor while maintaining safe clearance from the ground. Specifically, the invention is an accelerometer or inclinometer that is used in conjunction with an energized electrical conductor to sense inclination angle (relative to horizontal) of the suspended conductor at a given suspension point in real time and at regular intervals based upon a voltage output of the accelerometer that is dependent on its angle where the inclination angle is then used to calculate the sag of the conductor which is then used to determine the maximum allowable power transmission while still maintaining safe clearance between the energized electrical conductor and the ground or other obstruction.
2. Background Information
With deregulation of utilities including electrical utilities, it is now more important than ever that utilities be efficient in the delivery of services since competition now exists. In addition, deregulation has opened up new markets for individual utilities and as a result all utilities are seeking to expand while defending their home region. As a result, there is an ever increasing need for electric utilities to transfer more power through their existing power lines, that is to maximize transmission through existing resources.
It is well known in the power transmission industry that one of the major hurdles to increased power transmission is clearance between the power line and the ground or structure. Code mandated safety considerations for overhead or suspended power lines requires utilities to provide adequate clearances between the ground and/or structures under the power line. This clearance must be maintained at all times including in all weather conditions and under all actual load conditions.
This is one of the major considerations to electrical utilities because power lines sag under increasing power loads and as a result limitations are placed on the ampacity or maximum load a line is allowed to carry. Basically, the reason for this is the well known principal that power lines sag as load is placed on the power line and that sag increases as the load increases. This sag-load correlation is the result of heat causing thermal expansion of the conductor corresponding to load levels. In more detail, heat is generated in the conductor by the resistance losses resulting as electrical current flows through it. This heat causes thermal expansion of the conductor. As load increases more heat is generated resulting in ever increasing thermal expansion of the power line causing the power line to sag closer to the ground. Since government regulations mandate the minimum clearance, utilities must assure that this minimum clearance is never violated.
In addition, numerous other factors also affect the suspended power line and the sag therein including ambient temperature (warmer temperatures increase sag), and wind speed and direction (wind usually cools the line and thus decreases sag). All of these factors, and primarily the thermal expansion, are critical considerations to electrical utilities as indicated above because steps must be taken to assure the adequate clearances as required by law are maintained. As a result, ampacity or maximum load is generally limited to less than maximum levels as a safety factor to assure minimum clearance is maintained at all times and under all weather and load conditions. It is often typical that the safety is a significant factor and thus maximum load is significantly affected.
It is well known in the industry that such adequate clearance regulations are necessary because power lines, after being installed in relation to the ground or structures, later sag so as to become too close to the ground or structures resulting in significant safety concerns. One such concern is that when power lines sag too close to the ground, electrical shock or contact with the lines becomes more feasible and thus safety is at issue. Another such concern is that electric flashover scenarios are possible as lines become too close to electrically grounding objects such as the ground or structures, and such electric flashover is likely to result in extensive damage.
During installation and before a load is placed on the lines, the power lines can be installed such that sufficient clearance is achieved. This can readily be done by mere visual sight alignment or by simple measurement techniques measuring the distance from the lowest part of the line to the ground or nearest structure. It is even possible to very roughly account for factors such as ambient temperature, wind speed, wind direction and other environmental factors using conservative assumptions and historical knowledge. It is noteworthy though that such conservative assumptions result in significantly less than maximum line loading.
However, once an electrical load is placed on the power lines, various load factors cause the power lines to sag. Of these various factors, by far the most significant is thermal expansion of the power line under load as mentioned above, and specifically under a continuously varying load. It is well known that the clearance between a suspended electrical conductor and the ground decreases as the conductor sags due to this thermal expansion under load. Thermal expansion is directly correlated to load in the conductor such that increased load results in increased thermal expansion. Due to the desire to transmit as much power as possible through electrical conductors, this thermal expansion and the resulting sag is critical.
It has been realized that the full utilization of transmission lines requires proper analysis of sag and clearance with respect to these sag factors and most importantly the thermal expansion factor. In theory, this allows for the calculation of maximum load which still provides for minimum clearance as required by safety regulations. Current technology is such that several approximate methods provide for such approximation or calculation.
The first prior art method involves measuring the temperature of the conductor at a spot. Mathematical modeling is then used to calculate the sag. It has been found that this method is an approximation at best because line temperature varies based upon location radially within the line, location on the line, wind, exposure to elements, etc. and thus the approximate is often inaccurate.
Basically, safety factors are instituted to assure minimum clearances at all times thereby not optimizing the thermal expansion and sag allowed. Some of the problems of this method are due to its approximating qualities rather than accurate calculations. Other disadvantages and/or problems result from the inability to measure the temperature at all points, instead of sample points. As a result of these and other disadvantages and problems, significant safety factors are a necessity to assure minimum clearances but as a result optimization suffers.
Alternatively, the environmental factors have been measured on the spot and then used to calculate the actual conductor temperature in conjunction with the above mentioned conductor temperature reading. This approach is time consuming, labor intensive, indirect and often subject to large errors.
Numerous examples exist for monitoring sag using temperature including those disclosed in U.S. Pat. Nos. 5,235,861 and 5,517,864. In the background section of each of these patents, the early disadvantages of temperature measurement techniques are pointed out. These disadvantages include the highly conservative current ratings resulting from an assumed combination of worst cooling conditions. It is noted that these often include a combination of highest expected ambient temperature and lowest wind speed, all of which are rarely the actual conditions. These patents then describe the improvements each has made to the monitoring of sag using temperature, improvements including adding a time function to the calculation, that is to intermittently calculate rather than worst case scenario.
Another prior art method involves the actual measurement of conductor sag or alternatively the ground clearance. This has been done with actual measuring, using acoustics, microwaves, and laser beams, although none of these methods has proven to be practical. The equipment is often bulky and heavy. It is also expensive. The equipment is typically installed on the ground under the conductor and thus must be left unattended whereby it is subject to vandalism, and it reduces the clearance at the critical center portion of the line where it is installed.
Another prior art method involves measuring the power line tension at a suspension point. Since the line tension is affected by its inclination angle, by knowing the tension, the inclination angle can be determined and thus the sag. It has been found that certain limitations and/or disadvantages are associated with this tension measuring method. First, load cells used to measure the tension must be capable of measuring very small changes in a large static tension which is continuous on the line; and as a result, the accuracy of the sag determination is based upon the accuracy of the load cell and its capability of measuring small tension changes. Second, often load cells must be installed in-line which requires de-energizing and cutting of the line; and as a result, significant labor expense and line downtime is incurred. Finally, many of the current tension reading load cells must be installed on the grounded end of insulators holding the line at dead-end structures; and as a result, calculations cannot be performed on all spans.
Numerous examples exist for monitoring sag using tension including those disclosed in U.S. Pat. No. 5,454,272. The device described herein analyzes mechanical waves to determine the tension in a cable. Basically, the line is contacted with an impact stimulus with a force transverse to a length of the line thereby creating an incident vibrational wave in the line. The wave is propagated along the length of the line and propagation wave is sensed by a first accelerometer which detects the passing and amplitude of the wave. A second accelerometer spaced apart from the first accelerometer subsequently senses the passing and amplitude of the same wave. Tension is then calculated using these two measurements. A complex system and equipment is needed to perform this type of testing such as the one shown in the figures of this patent.
It is thus desirable to discover a simplified, more accurate, easy to use, time sensitive, system of monitoring sag in power transmission lines.